The study on anaerobic decomposed of lignin by black-liquor-philic fungus was carried out in this paper. The elemental components, molecular weight, fund and IR, 1H-NMR of lignin sample after anaerobic reaction were investigated. The result showed that in the anaerobic decomposition the lignin which molecular weight was bigger than 4950 was fragmented. The coumaran, toupentine structure and β-O-4, β-β link disappeared because of fragmentation. At the same time, demethoxylation and dehydroxylation took place in the anaerobic reaction, and dissolvability lower molecular compound was produced.